This invention relates to the purification of particulate glass obtained by froth flotation from municipal waste or trash, and is particularly concerned with procedure for removal of the deleterious refractory contaminants silicon carbide and aluminum oxide, particularly corundum, and especially as they appear in the form of relatively large size particles or stones, intermixed with froth-floated glass.
In the practice of recovering glass from shredded or incinerated municipal trash or waste by froth flotation, certain impurities are recovered with the glass. Iron as metals and oxide, often present in such recovered glass, has been removed heretofore by magnetic separation because it is an important color-causing impurity in glass. The U.S. Bureau of Mines, for example, has employed a high intensity wet magnetic separator on glass-containing incinerator residue for this purpose. Thus, the U.S. Bureau of Mines has employed a high intensity wet magnetic separator to remove weakly magnetic particles such as Fe.sub.2 O.sub.3 or FeO, in conjunction with the floation method for glass recovery. Although such high intensity wet magnetic separation is effective in removing most of such iron or iron oxide impurities from the glass, it is ineffective in removing stones of refractory materials. Refractory particles or stones are particularly deleterious in the glass, since such impurities will not dissolve in the molten glass, and will subsequently form stones in the finished glass article produced from the froth floated glass. In contrast, the presence of a small amount of iron left in the glass does not seriously affect the quality of the glass product.
Corundum is the most frequently occuring refractory found in municipal trash, with silicon carbide nearly as common. As previously noted, the use of a high intensity wet magnetic separator has been found ineffective in removing such refractories or stones from froth floated glass or glass concentrate. Thus, bottles made using only 5% of such magnetically purified glass contained over 50 such stones in each pound of glass product. This is substantially in excess of the amounts of such impurities permitted in glass cullet. Thus, the specification for glass cullet with regard to the presence and number of such stones is particularly stringent with respect to the number of coarser stones of a size smaller than 20 mesh (Tyler) and larger than 40 mesh (-20+40 mesh), being not more than 50 millionths of 1% by weight, and the number of smaller stones of a size of 40 to 60 mesh (-40+60) being 5 ten thousandths of 1% by weight. It is therefore apparent that wet magnetic separation is not effective in reducing refractories to a sufficiently low level permitted according to the specification for purity of glass cullet, especially with respect to impurities such as silicon carbide and corundum stones.
In my copending application Ser. No. 795,531, filed May 10, 1977, there is disclosed a process for effectively removing silicon carbide and corundum from glass of relatively fine size produced by froth flotation of municipal trash, by electrostatic separation, particularly high tension electrostatic separation.
It is an object of the present invention to provide novel procedure for efficiently removing silicon carbide and/or aluminum oxide, particularly corundum, impurities from glass by a procedure different from that of my above application. Another object is the provision of novel procedure for separating such impurities in relatively large particle sizes in the form of stones greater than 40 mesh size, e.g. 20 to 40 mesh stones, as well as smaller 40 to 60 mesh size stones, from glass obtained by froth flotation. Yet another object is to provide a novel procedure to obtain a relatively high purity glass, e.g. in conformance with the specification for glass cullet, from froth flotation glass containing silicon carbide and corundum impurities.